Chemistry Reference
In-Depth Information
numbers as a metric over others such as surface area or chemical composition.
Further concerns include lack of standardisation of the key measurement
parameters, including sampling, which is necessary for robust evaluation of
PNCs [
27
]. What should be the lower cut-off size for particles for ambient
regulations is another question. Adopting 23 nm lower cut-off size as in the
Euro vehicle emission standards will leave out over one third of total ambient
PNCs, as highlighted in above sections. Variations in PNCs can be easily up to
two orders of magnitude or more between the background and roadside
environments [
2
,
21
], raising a question about choosing an appropriate limit
value which can address this remarkable spatial variation seen at different
locations (i.e. at road, roadside, street canyons) within an urban area. A further
question can be raised on appropriate sampling height that can represent exposure
to the entire population living at a particular location (i.e. at ground floor or
above); past studies have shown appreciable changes in PNCs near the road level
within about the first 2 m and then decreased concentrations as move upward
from the road surface [
79
,
104
].
At present, there is no agreed safe threshold limit for exposure to ambient
nanoparticles due to the lack of a sufficient knowledge on the exposure-response
relationships, making developing any regulatory framework even harder. Vehicle
emissions can increase the PNCs up to an order of magnitude higher in urban
environments compared with natural environments, meaning that future control and
management strategies should target a decrease of PNCs in urban environments
by more than one order of magnitude which is not a trivial task [
21
]. Last but not
the least, other challenges can include an adequate treatment of PNC peaks which
can arise due to secondary particle formation, enhancing the complexity whether
the regulations should be set around the baseline PNCs without taking into account
the peak PNCs, or should include the peaks which needs to be defined first [
21
].
All the aforementioned questions warrant further research and some definite
answers before any regulatory framework is proposed.
8 Synthesis and Future Work
A considerable amount of development has happened in the last two decades in the
area of measurements, dispersion modelling and exposure assessment studies
related to airborne nanoparticles. This is clearly evident from the ever-increasing
number of published studies in Europe, and elsewhere in general. This study
presented PNCs over 45 sampling locations covering about 30 cities and
15 European countries. While reviewing the literature, it was felt that there are
still a number of European countries where nanoparticle-related studies are scarce.
Airborne nanoparticles empirically fit well to log normal distributions and
exhibit bimodal distributions in atmospheric urban environments. These arise
from both natural and anthropogenic sources. Road vehicles remain a dominant
source, contributing up to 90% of total PNCs, in polluted urban environments.
